1. Field of the Invention
This invention is related to the general subject of seismic exploration and, in particular, to the interpretation of seismic data.
2. Description of Related Art
For many years seismic exploration for oil and gas has been conducted by use of a source to generate seismic energy and the reception of the energy generated by the source by an array of seismic detectors. On land, the source of seismic energy may be a high explosive charge or another energy source having the capacity to deliver a series of impacts or mechanical vibrations to the earth""s surface. Acoustic waves generated by these sources travel downwardly into the earth""s subsurface and are reflected back from strata boundaries and reach the surface of the earth at varying intervals of time, depending on the distance traveled and the characteristics of the subsurface traversed. These returning waves are detected by the sensors, which function to transduce such acoustic waves into representative electrical signals. The detected signals are recorded for later processing using digital computers. Typically, an array of sensors is laid out along a line to form a series of detection locations. More recently, seismic surveys have been conducted with sensors and sources laid out in generally rectangular grids covering an area of interest, rather than along a single line, to enable construction of three dimensional views of subsurface reflector positions over wide areas. Normally, signals from sensors located at varying distances from the source position are added together during processing to produce xe2x80x9cstackedxe2x80x9d seismic traces. In marine seismic surveys, the source of seismic energy is typically air guns. Marine seismic surveys typically employ a plurality of sources and/or a plurality of streamer cables, in which seismic sensors are mounted, to gather three dimensional data.
Initially, seismic traces were used simply for ascertaining formation structure from displays of seismic data. However, in 1979, Taner et al. published the work Complex Seismic Trace Analysis, Geophysics, Volume 44, pp. 1041-1063 (1979), and exploration geophysicists have subsequently developed a plurality of time-series transformations of seismic traces to obtain a variety of characteristics that describe the traces, which are generally referred to as xe2x80x9cattributesxe2x80x9d. Attributes may be computed prestack or poststack. Poststack attributes include reflection intensity, instantaneous frequency, reflection heterogeneity, acoustic impedance, velocity, dip, depth and azimuth. Prestack attributes include moveout parameters such as amplitude-versus-offset (AVO), and interval and average velocities. Further, attributes may be categorized as either instantaneous attributes, wavelet attributes or geometrical attributes. Instantaneous attributes are attributes whose values are obtained for each data point in the seismic data or within a small time window of data points (e.g., a few milliseconds), such as amplitude, phase, frequency and power. Wavelet attributes are the instantaneous attributes computed at the maximum point of the envelope and the physical meaning of all the wavelet attributes is essentially the same as their instantaneous counterparts. Geometrical, or interval, attributes are attributes of a seismic trace within a seismic interval which are computed from the reflection configuration and continuity.
In U.S. Pat. No. 5,226,019, which issued on Jul. 6, 1993, to Michael S. Bahorich, it is stated in column 3 that with reference to seismic attributes xe2x80x9ccombining multiple (i.e. two or more descriptors through addition, subtraction, multiplication and ratio, or other means can also be successfully employedxe2x80x9d, and suggests use of xe2x80x9ca product of the average instantaneous amplitude and average instantaneous frequencyxe2x80x9d.
U.S. Pat. No. 5,884,229, which issued on Mar. 16, 1999, to Gianni Matteucci, discloses a statistical method for quantitatively measuring the lateral continuity of the seismic reflection character of any specified location in a subsurface target formation.
U.S. Pat. No. 5,930,730, which issued on Jul. 27, 1999, to Marfurt et al., discloses a system for forming a seismic attribute display from calculated measures of semblance and corresponding estimates of true dip and true dip azimuth of seismic traces within an analysis cell.
U.S. Pat. No. 6,012, 018, which issued on Jan. 4, 2000, to William I. Hornbuckle, relates to a system for identifying volumetric subterranean regions bounded by a surface in which a specific seismic characteristic has a constant value. It is stated at column 3, line 36 that, xe2x80x9cin a geological region where physical characteristics (e.g., the presence of oil or gas) are well-correlated with seismic attributes, (e.g., seismic amplitude data), the identification of a subvolume bounded by a constant-seismic-attribute-value surface may provide a very useful predictor of the volumetric extent of the attribute and hence of the characteristic.xe2x80x9d
U.S. Pat. No. 5,001,677, which issued on Mar. 19, 1991, to A. Ronald Masters, discloses a system which treats measured attributes derived from seismic data as components of a vector, estimates a background vector representing typical background geologic strata, and then calculates a new attribute. As stated on col. 8, line 11, the preferred embodiment combines information about P and S impedance contrasts so as to discriminate prospective reservoir strata from surrounding non-reservoir or background strata.
U.S. Pat. No. 5,724,309, which issued on Mar. 3, 1998, to Higgs et al, discloses a system in which two new seismic attributes (dip magnitude and dip azimuth) are derived from instantaneous phase. The system comprises determining a spatial frequency value by taking the directional spatial derivative of the instantaneous phase for each of a plurality of x,y,t(z) data points in the seismic data and posting the spatial frequency values to identify changes within the earth""s subsurface.
U.S. Pat. No. 5,870,691, which issued on Feb. 9, 1999, to Partyka et al., discloses a method for processing seismic data to identify thin beds.
Although it is generally recognized that specific seismic attributes are related to specific subsurface properties, a need continues to exist for advancements in the use of seismic attributes to improve the delineation of subsurface regions of the earth to assist in the exploration and production of oil, natural gas and other minerals.
It should be noted that the description of the invention which follows should not be construed as limiting the invention to the examples and preferred embodiments shown and described. Those skilled in the art to which this invention pertains will be able to devise variations of this invention within the scope of the appended claims.
The invention in a first embodiment comprises a method of utilizing seismic data attributes for interpreting seismic data from a region of the earth""s subsurface, in which values are calculated for a plurality of seismic data attributes of said seismic data, and combinations of said calculated values are generated to develop an indication of shaliness of said region of the earth""s subsurface.
In another embodiment the invention comprises a device, which is readable by a digital computer, having instructions thereon for defining a process and instructing a computer to perform a process for calculating values for a seismic data attribute of said seismic data indicative of thinness of subsurface strata, calculating values for a seismic data attribute of said seismic data indicative of parallelism of subsurface strata, calculating values for a seismic data attribute of said seismic data indicative of lateral continuity of subsurface strata, calculating values for a seismic data attribute of said seismic data indicative of continuity consistency of subsurface strata, and generating combinations of calculated values for said plurality of seismic data attributes to generate an indication of shaliness of said region of the earth""s subsurface.